One shape known in the art and widely employed to date for both soft contact lenses and hard contact lenses (hereinafter referred to collectively as “contact lenses”) is that depicted in FIG. 6(a), which has a back surface optical zone 50 formed in the center portion of the back surface of the lens and a back surface peripheral zone 52 formed along the outer circumference of this back surface optical zone 50, while having formed in the center portion of the front surface of the lens a front surface optical zone 54 and a front surface peripheral zone 56 formed along the outer circumference of this front surface optical zone 54, thereby forming an optical zone 58 in the center area of the lens and a peripheral zone 60 situated in the peripheral area of the lens.
When designing the shape of such a contact lens 62, the typical procedure includes: to first establish the diametric dimension: Doz of the back surface optical zone 50 and the diametric dimension: Dpz of the front surface optical zone 54, as well as designing the shape of the back surface optical zone 50 and back surface peripheral zone 52 having a base curve of generally spherical shape conforming the shape of the wearer's cornea; to then establish lens thickness: t1 in a front surface junction (i.e. the connecting zone of the front surface optical zone and front surface peripheral zone) 64; and to then design the shape of the front surface optical zone so as to give the required diopter power, as well as establishing appropriate shape for the front surface peripheral zone on the basis of a function or arbitrary curve, so that a front surface junction 64 and an edge portion (peripheral edge of the lens) 66 join up smoothly.
Meanwhile, if the peripheral zone of a contact lens is too thin, it becomes difficult for the lens to consistently hold its shape, and particularly in the case of a soft contact lens may make it difficult to distinguish the front from the back, which can pose a risk of problems in handling. Also, since contact lenses are generally not provided on a purely “order-made” basis to wearer but are rather provided from a series prepared and provided as a combination lenses with optical zones that come in predetermined suitable number of diopter powers, with the lenses most suitable for the wearer being selected from the series. Thus, for each contact lens in the series having different front surface optical zone shapes in order to provide different diopter strengths, it is necessary to design lenses in such a way as to ensure consistent shape retention.
Thus, the typical practice in the past for a contact lens 62 like that illustrated in FIG. 6(a) having generally uniform thickness throughout the entire optical zone and low diopter power (for example, −1.0 diopter power) was to appropriately design the shape of the front surface peripheral zone 56 and the thickness dimension: t1 at the location of a front surface junction 64 so as to enable the lens to consistently hold its shape through the rigidity of the peripheral zone 60, thereby designating this as the base lens shape, and to then design for this base lens shape the front surface optical zones 54 that give different diopter powers. When using this design method, a predetermined acceptable minimum thickness is established with reference to the lens material and the like, and in the event that the lens center thickness is to thin owing to the diopter power of the front surface optical zone 54, the thickness of the front surface junction 64 and peripheral zone 60 is redesigned to be thicker, so that thickness at the lens center is at least equal to predetermined minimum thickness.
Specifically, where the required diopter power is more positive than that of the base lens, or where more negative but the absolute value of diopter power is small, the front surface optical zone 54 can be designed making the front surface junction 64 thickness and peripheral zone 60 shape generally uniform. Whereas if the diopter strength is much more negative than that of the base lens (e.g. −15 diopters), by redesigning lens thickness in the front surface junction 64 and peripheral zone 60 (in particular, a back surface junction 66 thickness: t2) to be greater than the base lens as shown in FIG. 6(b), for example, lens center portion thickness: t0 is obtained to be sufficient to ensure that there are no problems in terms of strength or durability. According to such design methods, none of the contact lenses in a series will have peripheral zone lens thickness smaller than that of the base lens shape, so that each of them can considerably hold lens shape on the basis of the rigidity of the peripheral zone.
In contact lenses of a series designed on the basis of the design method described above, depending on the diopter power established for the optical zone 58, and in particularly where diopter power is significantly negative, it becomes necessary to design and manufacture lenses that differ in shape according to the magnitude of diopter power, not just in the front surface optical zone 54, but over the entire front surface of the lens including the front surface peripheral zone 56. A resultant problem was that design and manufacture of contact lenses making up a series was tedious.
Additionally, research conducted by the inventors has shown that where contact lenses of series based on prior art design methods are provided, when diopter power differs among contact lenses of a given series, the thickness dimension of the peripheral zone 60 and mass associated therewith differ appreciably, which in some cases can result in differences in the movement of the contact lens over the cornea due to the action of the eyelid occurring with blinking, or the position of rest of the contact lens on the cornea due to the effect of gravity or the like, resulting in some instances in adverse effects on wear comfort and vision.
With the foregoing in view, it is therefore an object of the present invention to address these problems by providing contact lenses in a series composed of a combination of contact lenses of novel structure, whereby even in cases where diopter power or other optical characteristics differ, consistently good wear comfort can be achieved, while at the same time ensuring adequate shape retaining action by the peripheral zone and strength at the lens center.
It is a further object of the invention to provide a contact lens of novel shape effective for creating contact lenses of a series.
It is yet a further object of the invention to provide a novel method of manufacturing contact lenses, whereby contact lenses of a series may be manufactured efficiently.
Specifically, while various shapes for the back surface peripheral zone have been studied to date in consideration of tear fluid exchange and stability, the shape retaining effect of the peripheral zone thickness dimension in soft contact lenses has received only a cursory examination and is not viewed as particularly important. Research conducted by the inventors, however, has shown that the shape of the peripheral zone has significant effects on contact lens wear comfort and vision, and in particular that appreciable thickness of the peripheral zone including the front surface junction can result in appreciably impaired wear comfort owing to pressure applied by the eyelid and the like, as well as appreciable movement of the lens over the cornea by the eyelid during blinking, or a tendency of the lens rest position to shift vertically downward on the cornea due to the greater effect of gravity, or in other such problems.